1. Field of the Invention
This invention relates generally to memory systems and more particularly to a system and method for generating position error signals within a magneto-optical computer memory device.
2. Description of the Background Art
Efficient and economic storage of digital information is an important consideration of manufacturers, designers and users of computing systems. In magneto-optical storage devices, digital data is typically stored in tracks located on rotating disks of MO storage media. Close positioning of the adjacent disk tracks maximizes the amount of stored data on a storage disk, thus providing significant economic benefits to system manufacturers and users. Therefore, system designers frequently seek new and improved methods of. reducing track pitch to permit greater storage capacity on the storage media.
Referring now to FIG. 1(a), a plan view of a front surface 112 of a magneto-optical storage media 110 is shown. In magneto-optical storage devices, digital data is typically written into and read from a series of concentric or spiral tracks 114 located within a plurality of data wedges 177 on the surface 112 of storage media 110. In practice, the digital data is read from the front surface 112 of storage media 110 by projecting a laser-generated light spot from a flying read/write head onto a selected track 114 while storage media 110 is rotating, and then sensing the polarization of light reflected back from storage media 110.
The read/write head must be accurately positioned above track 114 of rotating storage media 110 during a read/write operation on that track. Many factors (for example, imperfections in track symmetry) may cause the read/write head to be positioned slightly off the center of track 114, thus requiring position correction of the head for acceptable performance during a read/write operation. One prior art position correction method utilizes a diffraction pattern to generate a position error signal from grooves that are positioned between tracks on the media. Another correction technique is the use of pre-patterned media with position marks embossed on the tracks within a plurality of servo sectors 178 to generate a position error signal (PES). The PES may then provide feedback to compensate for position errors by adjusting the radial position of the read/write head.
Referring now to FIG. 1(b), a diagram of position marks on sample storage media tracks within a servo sector is shown. FIG. 1(b) includes sample tracks 1 (120) through 5 (128). In FIG. 1(b), five tracks are presented for purposes of illustration, however storage media 110 typically contains a significantly larger number of tracks. Furthermore, FIG. 1(b) depicts track 1 (120) through track 5 (128) as being straight, whereas in practice they are typically circular. As shown in FIG. 1(b), each track 1 (120) through 5 (128) has three associated position marks which may be repeated at selected intervals along their corresponding track. The position marks are formed by depressions in the surface 112 of storage media 110 and effectively reduce the reflectivity of surface 112 to thereby attenuate light reflected back to the read/write head from within a full width half maximum diameter of an optical spot 154 formed by an impinging beam of light. Since the operation of each track is similar, track 5 (128) will be used in conjunction with FIG. 1(c) to describe the function of respective position marks 140, 142 and 144.
Referring now to FIG.1(c), a drawing of a reflectivity waveform corresponding to position marks 140, 142 and 144 (FIG. 1(b)) is shown. During a read/write operation on track 5 (128), the read/write head is positioned over track 5 (128) as media 110 rotates at a selected rate of speed. The read/write head initially encounters position mark 140 which is centered on track 5 (128) and which then generates a sync pulse 162 at time 164.
Next, the flying head encounters position mark 142 which is positioned at a specified perpendicular distance "D" off-center of track 5 (128), in the direction of track 4 (126). Position mark 142 then generates a pulse "A" 166 at time 168. The amplitude of pulse A 166 is relatively less than the amplitude of sync pulse 162. Then, the read/write head encounters position mark 144 which is positioned at the same specified perpendicular distance "D" off-center of track 5 (128), but in the opposite direction of position mark 142. Position mark 144 then generates a pulse "B" 170 at time 172. The amplitude of pulse B 170 is also relatively less than the amplitude of sync pulse 162. The radial position error signal (PES) for the read/write head may thus be obtained by taking the difference of the peak reflectivity amplitudes of pulse A 166 and pulse B 170. The separation of the edges of position marks 142 and 144 determines the linearity of the PES.
In prior art storage systems, the optimal diameter of position marks is equivalent to the full width half maximum (FWHM) value with an optical spot formed by an impinging read/write laser beam, and the distance between adjacent tracks is typically two times this FWHM diameter. FIG. 1(b) illustrates an intensity profile 159 of the light spot and the width 156 of the light spot at the FWHM value. In the prior art, spacing between adjacent tracks is also limited by the size and pattern of the position marks. The limit on increased spacing between adjacent tracks reduces the maximum data density available from the storage media. What is needed, therefore, is an improved system and method that overcomes the aforementioned limitations of the prior art.